1. Field of the Invention
The invention relates generally to the field of marine seismic data acquisition and interpretation. More specifically, the invention relates to methods for correcting marine seismic data for the effects of receiver movement during data recording after actuation of a seismic energy source.
2. Background Art
Seismic surveying is used to evaluate structures of, compositions of, and fluid content of subsurface earth formations. A particular application for seismic surveying is to infer the presence of useful materials, such as petroleum, in the subsurface earth formations. Generally, seismic surveying includes deploying an array of seismic sensors at or near the earth's surface, and deploying a seismic energy source near the sensors also at or near the surface. The seismic energy source is actuated and seismic energy emanates from the source, traveling generally downwardly through the subsurface until it reaches one or more acoustic impedance boundaries. Seismic energy is reflected from the one or more impedance boundaries, where it then travels upwardly until being detected by one or more of the sensors. Structure and composition of the subsurface is inferred from the travel time of the seismic energy, and the amplitude and phase of the various frequency components of the seismic energy with respect to the energy emanating from the seismic source.
Marine seismic surveying techniques known in the art include towing a seismic energy source behind a ship, and towing one or more arrays of seismic sensors (“streamers”) behind the same or a different ship, along the surface of a body of water. The seismic energy source may be an air gun or array of air guns, a water gun, explosives any or other type of seismic energy source well known in the art. A streamer generally consists of a long cable having seismic sensors, typically hydrophones, disposed thereon at selected positions along the cable. The streamer generally extends behind the ship and along the direction of motion of the ship. If more than one streamer is used in a sensor array, the streamers include equipment designed to hold the streamers at laterally spaced apart positions from each other.
The seismic survey ship typically includes a data recording system. The data recording system includes a device for controllably actuating the seismic energy source, equipment for determining the geographic position of the ship at any moment in time (typically using global positioning system (GPS) navigation devices), and equipment for recording the signals generated by the seismic sensors in response to seismic energy reflected from the subsurface earth formations. The recordings of the signals are typically indexed with respect to each time of actuation of the seismic energy source. The GPS navigation equipment may also include devices to determine the geographic position of each sensor on each streamer at any selected time.
During a marine seismic survey, the ship moves along a selected path, called a “sail line” through the water. At selected times, the seismic energy source is actuated, and the signals detected by the seismic sensors are recorded with respect to time. At the time of actuation of the source, the geographic position of the ship, the seismic source and each sensor on the one or more streamers is recorded. To interpret the recorded signals to infer structure and composition of the subsurface earth formations, it is necessary to know the geographic position of the source and each sensor at the time the signals were recorded. This is both to assure that seismic signals correspond to determinable geographic positions with respect to the earth's surface, and to enable various forms of signal correlation and stacking, such as common mid point (CMP) stacking to be properly performed. During acquisition of seismic data, however, the ship, the source and the streamers are moving along the water surface. As a practical matter, the position of the source at the time of source actuation can be used as the actual position of the source, because the duration of the energy impulses generated by the source is short enough such that the ship, source and streamers do not move a substantial amount during any individual source actuation. However, seismic energy can take several seconds to travel through the water, through the subsurface earth formations, reflect upwardly and travel back through the formations, the water and to the seismic sensors. Therefore, the sensors may move a substantial distance between the time of actuation of the source and the end of signal recording for any individual source actuation. The amount that the sensors move depends on the velocity of the ship, the dynamic stretching of the cable, the deflection of the cable due to currents, and the two-way travel time of the seismic energy from the source to each of the sensors. As a result, the geographic position of the subsurface acoustic impedance boundaries corresponding to the detected seismic energy change during the recording.
There are methods known in the art for correcting seismic data recordings for movement of the seismic sensors during recording. One example of such methods is disclosed in U.S. Pat. No. 6,151,556 issued to Allen. The method disclosed in the Allen '556 patent is primarily intended for use with marine vibrator sources, which produce a seismic energy pulse over a period of several seconds. The method includes correcting signals for the movement of the seismic source, correcting signals for the movement of the receivers and combining the corrected data. The method in the Allen '556 patent assumes that the motion of the source and the receivers is substantially the same as the motion of the seismic vessel.
U.S. Pat. No. 6,480,440 issued to Douma et al. discloses another method for correcting seismic data for receiver motion. The method disclosed in the '440 patent includes determining an offset between the seismic source and a seismic receiver, determining a normal moveout velocity of the seismic energy between the source and the receiver, and determining a corrected arrival time of reflected seismic energy based on the normal moveout velocity and the velocity of the seismic vessel. A limitation of the methods disclosed in the foregoing references is that they only account for movement of the sensors as being directly related to the velocity of the seismic vessel. In some instances, the sensors may also move transversely with respect to the sail line due to, for example, currents in the water. It is also possible that the component of the sensor velocity along the sail line does not precisely match the velocity of the seismic vessel due to stretching of the streamer cable. It is therefore desirable to have a method to correct seismic data for movement of the sensors that takes account of the sensor velocity in both the sail line and transverse directions.